When silicon (Si) is added to aluminum (Al), remarkable effects are attained for reducing the thermal expansion coefficient, increasing the Young's modulus, improving the wear resistance, and the like. Al-Si alloys utilizing such effects have already been widely used.
Among such Al-Si alloys, a cast material is classified as AC or ADC under the Japanese Industrial Standards, and widely used as an aluminum alloy for casting for example engine blocks. An Al-Si alloy prepared as an wrought material is classified in the 4,000 series, and worked into various parts by extrusion, forging or the like starting with a cast billet.
It is well known that a hyper-eutectic Al-Si alloy has been prepared by a casting method. A hyper-eutectic Al-Si alloy casting obtained by the casting method, which has excellent properties such as a low thermal expansion coefficient, a high Young's modulus and a high wear resistance, is expected to be used in various fields. When such a hyper-eutectic Al-Si alloy casting contains coarse primary crystals of silicon, however, its mechanical properties and machinability are deteriorated.
It is also well known that a refiner, particularly phosphorus (P), may be added in order to refine the primary crystals of silicon contained in a hyper-eutectic Al-Si alloy casting. Even if such a refiner is added when a hyper-eutectic Al-Si alloy is cast, however, the refinement of silicon primary crystals is restricted. Particularly when the Al-Si alloy contains silicon in excess of 20 percent by weight, coarse primary crystals of silicon still remain even if the refiner is added, and hence the mechanical properties and machinability of the alloy are still deteriorated.
In recent years, on the other hand, it is possible to prepare powder from a molten metal at a high cooling rate, which has been unavailable in a casting method, by a method of preparing a rapidly solidified powder, such as by atomizing. Therefore, primary crystals of silicon can be so refined that it is possible to prepare a hyper-eutectic Al-Si alloy powder containing silicon in excess of an eutectic composition and further containing a transition metal element X such as iron (Fe), nickel (Ni), chromium (Cr), manganese (Mn) or the like as a third alloy component. Powder metallurgical alloys such as Al-17Si-X, Al-20Si-X and Al-25Si-X, having properties even superior to those of cast alloys of this type have been put into practice as alloys prepared by a powder metallurgical method using such powder materials.
In order to further improve the mechanical properties of the aforementioned powder metallurgical alloys, it is necessary to further refine the crystals of silicon while simultaneously homogenizing the crystal grain sizes of silicon. Further, it is extremely important to reduce the presence of coarse crystals of silicon, which serve as starting points of rupture even if the amount thereof is small to cause a reduction in the material strength. In addition, such primary crystals of silicon contained in the powder can hardly be refined by hot solidification such as forging or extrusion, but rather become coarse due to Ostwald growth. Thus, the sizes of the silicon primary crystals contained in the alloy powder are definitely important.
It is known that a cooling rate in the preparation of the powder may be increased in order to refine primary crystals of silicon. However, such a cooling rate is generally obtained by a method of and an apparatus for atomizing, and no other industrial method of increasing such a cooling rate has been implemented due to economic problems especially a low productivity.
In the conventional atomizing method, the particle sizes of silicon primary crystals contained in the entire powder volume are extremely dispersed so far as the obtained powder has a particle size distribution of a constant width, since the cooling rate depends on the particle size of the powder. For example, powder of about 400 .mu.m in particle size has generally unavoidably contained coarse silicon primary crystals of about 20 .mu.m in particle size.
To this end, coarse powder having a low cooling rate has generally been removed by sieving or sifting in order to eliminate particles of coarse silicon primary crystals, thereby preparing consolidates including only fine powder particles. According to the known method, however, the material yield is reduced, making the known method economically unattractive. Further, the flowability of the; compactibility of the powder is substantially reduced and there is an increased danger of a dust explosion.